In business communication systems, a number of telephone stations are individually connected to a single controller which, in turn, connects to a telephone switching office. Such controllers have the job of supplying DC power to, and exchanging voice and data signals with, each telephone station in an efficient manner. It is desirable in these systems to minimize the number of wires between each station and the controller while keeping the voice and data signals from interfering with each other. Such needs, however, are frequently incompatible, and certain systems use up to four wire-pairs to achieve minimum interference between the signals.
In one well known technique, both voice signals and DC power are transmitted over a single wire-pair that is transformer coupled to the controler at one end of the wire-pair and to the telephone station at the other. A capacitor is placed in series with a pair of the transformer's secondary windings that interconnect with the wire-pair so that DC power will flow toward the station over one conductor of the wire-pair and return to the controller over the other. It is in this manner that standard telephone sets receive voice signals and DC power. However, DC power variations are not necessarily correlated with the voice signal, and since this particular technique causes the DC power to flow in the same direction through each secondary winding, the flux created in these windings additively combines. Power variations therefore create signals on the wire-pair, unrelated to the voice signal, that interfere with voice communication. Such signals are deemed to be "noise." One way to eliminate this noise is to use a current source; however, this adds cost to the station and places an unnecessary burden on the power supply in the controller.
It is therefore an object of this invention to provide an arrangement for delivering power and exchanging voice and data signals between a controller and a telephone station in a manner that minimizes mutual interference.
It is another object of the present invention to allow DC signaling in order to minimize the complexity of data transmitting and receiving circuitry.
It is yet another object of the present invention to minimize the number of wires between the controller and each telephone station needed in the distribution of power, voice, and data signals.